Synchronized controlled oscillation modulator

ABSTRACT

A synchronized controlled oscillation modulator (SCOM), comprising at least one, Controlled Oscillation Modulator ( 5 ), and synchronizing means ( 1, 6 ) connected to said COM modulator. The modulator can comprise several COM modulators, and the synchronizing means can be arranged between the COM systems, so that the oscillation modulator signal is used as synchronization signal. The invention provides significant advantages in performance, topological simplification, improved robustness, stability and efficiency compared to prior art.

TECHNICAL FIELD

This invention relates to a self-oscillating modulator, comprising acomparator and a power amplification stage for pulse width modulation,and higher order oscillating loops comprising first feedback means andfirst forward means securing stable oscillating conditions.

The invention also relates to a switching power conversion system, suchas DC-AC (e.g. audio amplification), DC-DC or AC-AC conversion systemsor any combination of the above mentioned comprising such a modulator.The invention may advantageously be used for improved power conversionin any system, in particular precision DC-AC conversion systems such ashigh efficiency audio amplification.

TECHNICAL BACKGROUND

The pulse width modulator is a central element of any power conversionsystem. Most switching power converters are based on Pulse WidthModulation (PWM) as means to control efficient conversion betweendomains (DC or AC).

A typical power converter may include a PWM modulator, a switching powerconversion stage, a filter and a control system. A prior art system ofthis type is described in U.S. Pat. No. 4,724,396 and by Mr. Attwood inJournal of the AES, November 1983. p. 842–853. However, PWM has a rangeof shortcomings also well known to the art, mainly due to theimplementation of the carrier generation. This limits the systembandwidth and complicates design. Also, a stable and robust controlsystem design is difficult.

In order to overcome these drawbacks, a controlled oscillating modulator(COM) was introduced in the applicant's international patent applicationPCT/DK97/00497. The disclosed modulator eliminates the need of a carriergenerator, with a range of advantages, described in detail in saiddocument.

A problem with this technique is that it can only synthesize standardtwo level modulation—hence giving disadvantages concerning theefficiency of the amplifier.

Another problem arises in multi channel systems such as multi channelaudio amplifiers, the oscillating modulators will have oscillationfrequency variations, which will cause intermodulation products, addingdistortion components within the audio bandwidth. A prior art system forsynchronizing an oscillating modulator to an external clock is given inU.S. Pat. No. 6,297,693. This prior art system can only comprise asawtooth or a triangular signal shape as synchronization signaleliminating possibilities to use COM modulator signals assynchronization signals. Furthermore the system can only synchronize amodulator to an external clock leading to higher complexity whenimplementing an external clock generator.

In multilevel systems such as, but not limited to PSCPWM systems (asdescribed in applicants international patent applicationPCT/DK98/00133), the first harmonic of the carrier is not present at theoutput, and said COM modulator can thereby not be used.

OBJECTS OF THE INVENTION

Accordingly, an object of the invention is to provide a superiormodulation technique in switching power conversion systems thatovercomes fundamental problems related to conventional techniques.

SUMMARY OF THE INVENTION

These objects are achieved by a novel synchronized controlledoscillating modulator (SCOM) of the type mentioned above, havingsynchronizing means connected to said COM modulator.

The invention provides significant advantages in performance,topological simplification, improved robustness, stability andefficiency compared to prior art.

The invention provides synchronization between a signal source and asingle COM modulator or a plurality of COM modulators and a signalsource or between a plurality of COM modulators in order to overcomeprior art problems related to COM modulators being desynchronized.

The COM modulators can comprise voltage or current measurement means,and feedback.

The synchronization means can use an external source as synchronizationsignal, where the external source can preferably but not necessarily bea triangular-, square- or sinusoidal signal.

Alternatively, the modulator comprises several COM modulators, and thesynchronizing means are arranged between the COM systems, so that theoscillation modulator signal is used as synchronization signal. In thiscase, the SCOM aims to combine the advantages of the COM technology withthe advantages of multi-level PWM.

According to this embodiment, pulse modulation in general powerconversion systems is provided that implements multiple level pulsemodulated signals—hence reducing the output switching noise energy andenhancing the possibilities for control system implementation.

The SCOM modulator according to the invention is very suitable in alltypes of precision DC-AC conversion applications as audio amplificationand motor or electrodynamic transducer drive applications.

The SCOM can advantageously be used in precision voltage or currentcontrolled DC-AC conversion as e.g. power amplifiers for audio use.

The power amplification stage can comprise an output filter, and thesecond feedback means can then be connected to an output from saidoutput filter. This permits a first filtering of the voltage before itis fed back in the feedback path.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention will be further describedin the following, with reference to the appended drawings.

FIG. 1 illustrates a prior art controlled oscillating modulator based onvoltage feedback.

FIG. 2 illustrates a prior art controlled oscillating modulation systemcomprising current feedback.

FIG. 3 illustrates a block diagram of a modulator according to a firstembodiment of the invention.

FIG. 4 illustrates a block diagram of a 3-level modulator according to asecond embodiment of the invention.

FIG. 5 illustrates a hardware implementation of the synchronizationmeans in FIGS. 3 and 4.

FIG. 6 illustrates a further embodiment of the invention for activesynchronization of COM modulators.

FIG. 7 illustrates an implementation of the active synchronization inFIG. 6.

FIG. 8 illustrates a power conversion system with two COM modulators,synchronized according to the invention.

FIG. 9 illustrates the open loop gain of the system in FIG. 8.

FIG. 10 illustrates an implementation of a synchronization according tothe invention in a PSCPWM modulator structure.

FIG. 11 illustrates a plurality of N (where N is an integer) COMmodulators synchronized by an additional synchronization signal.

FIG. 12 illustrates a plurality of N (where N is an integer) COMmodulators synchronized by their common COM signals.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

In the following detailed description of the preferred embodiments, theCOM modulators can be Voltage Controlled Oscillating Modulators (FIG. 1)as described in U.S. Pat. No. 6,297,692 or Current controlledOscillating Modulators (FIG. 2) as described in the InternationalPublication Number WO 02/025357, both hereby incorporated by reference.

A power conversion system comprising a modulator according to a firstpreferred embodiment of the invention is shown in FIG. 3. The systemcomprises a power stage 2, a control system with a feedback block 3 anda forward block 4. The power stage 2 can comprise one or a plurality ofhalf-bridges, preferably a full-bridge comprising two half-bridges. Thefeedback block and the forward block constitute an oscillating modulator5. An external signal source 1, also referred to as an oscillatingsignal generator block, is connected to a synchronization block 6 in themodulator 5.

The synchronization of the modulator 5 is obtained by adding the OscSignal from the source 1 to the modulating signal. The synchronizationsignal can be based on a voltage signal or a current signal depending onthe type of modulator (voltage based feedback or current basedfeedback). The Osc Signal can be a sinusoidal signal or any otheroscillating signal with the frequency of the wanted idle switchingfrequency and is used for synchronizing said modulator with said Oscsignal of the Osc Signal generating block 1.

A second preferred embodiment of the invention is shown in FIG. 4, wheretwo COM modulators 10, 11 are synchronized by one synchronizing signalfrom a synchronization block 12. Again, the synchronization signal canbe based on a voltage signal or a current signal depending on the typeof modulator (voltage based feedback or current based feedback).

The two COM modulators are designed to oscillate at almost the samefrequency, but a variation in the switching frequency can be eliminatedby the synchronization means.

By the use of two COM modulators 10, 11 it is possible to make a 3-levelmodulator. Each side of a load 13 is supplied with its own 2-level COMmodulator 10, 11, which each are synchronized.

The first harmonic of the carrier will ideally be eliminated across theload. The input signal is inverted by an inverter 14 preceding thesecond COM 11, in order to be able to make a differential audio signalon the output. The spectral characteristics resemble those of the NBDDmodulation. The NBDD can be seen from “Audio power amplifier techniqueswith energy efficient power conversion”, Ph.D Thesis by Karsten Nielsen.

The use of said 3-level modulation can advantageously be used fordriving a pulse modulated transducer directly without any outputfiltering of the PWM signal thus reducing eddy current losses in thetransducer 6 compared to a 2-level modulation.

FIG. 5 shows an example of a realization of the synchronizing means inFIGS. 3 and 4. The synchronization means are implemented as a circuit oftwo series resistances RA, RB connected to each end of a parallelcircuit consisting of a resistor ROSC and a capacitor COSC.

By the use of this network a small amplitude signal is added to the COMmodulator forcing the COM modulator to oscillate at the added smallamplitude signal frequency. Thereby one or a plurality of COM modulatorscan be synchronized by adding a small amplitude signal with thefrequency of the wanted idle frequency to each of the modulators.

The values of the resistances and capacitors can be determined by theskilled person in a trade-off between having good synchronization andnot having to influence either of the modulators negatively.

A further preferred embodiment of the invention is shown in FIG. 6 andillustrates an active synchronization system for obtainingsynchronization of COM modulators. The active synchronization block 15comprises one or a plurality active high pass filters.

FIG. 7 gives an example of the active synchronization block 15 in FIG.6, implemented as two active high pass filters 16, 17. One high passfilter 16 sums a first high pass filtered COM modulator signal at node18 to a second COM modulator signal at node 19 and thereby synchronizesthe second COM modulator with the first. The optimal synchronizationwill be obtained if also a second high pass filter 17 sums a high passfiltered second modulator signal at node 19 to the first modulatorsignal at node 18.

A power conversion system with two COM modulators, synchronizedaccording to the invention, is shown in FIG. 8. Two lag-blocks B1, B2are inserted in the forward path contributing to a higher loop gain atlow frequencies. There is not any high power filtering applied to thesystem but the load, preferably an electro-dynamic transducer, will actas an inductive load obtaining some filtering of the PWM signal. Therebythe output filter can be eliminated and efficiency increased. Each powerstage 20, 21 can comprise one or a plurality of half-bridges, andpreferably comprise a single half-bridge.

The open loop gain for the system in FIG. 8 is shown in FIG. 9. Thesystem is designed for a switching frequency of approximately 325 kHz.At 325 kHz the open loop gain is 0 dB and at that frequency the phase is−180 degrees, obtaining a controlled oscillation. The system shown inFIG. 8 will be capable of suppressing noise and distortion within the325 kHz bandwidth.

The system output is a differential 3-level PWM signal with highfrequency spectral characteristics resembling those of the NBDDmodulation obtaining a more efficient modulation compared to modulationtopologies with a differential two level PWM output signal.

If the modulators in the system shown in FIG. 9 are completelysynchronized, there will be a differential output of zero magnitude atidle. This is caused by the signal at idle on one terminal 22 of theload is equal to the signal on the other terminal 23 of the loadobtaining a differential signal of zero magnitude.

The synchronization can be obtained by synchronization means as shown inFIG. 5, as a synchronization network comprising an R, C or RC circuit.The R, C or RC circuit being connected to the comparator in the forwardpath. The synchronization can also be obtained as in FIGS. 6 and 7, asan active network comprising high pass active filter networks.

Furthermore the modulation depth can be controlled by limiting theamplitude of the input signal at the input signal node 24, achievinglower ripple currents.

FIG. 10 shows an SCOM according to the invention implemented inmultilevel PWM comprising a PSCPWM modulator structure and a MECC (N,M)control system where N,M are integers. MECC (N,M) is described in U.S.Pat. No. 6,297,692, hereby incorporated by reference. The systemcomprises one or a plurality of feedback paths and low pass filtering ofthe output PWM signal 25, 26. With the SCOM system comprising a PSCPWMmodulator it is possible to obtain a multi-level (more than two levels)modulator, preferably without high frequency common mode components onthe output. Each power stage 27, 28 comprises one or a plurality ofhalf-bridges. If each power stage 27, 28 comprises two half-bridges in afull-bridge structure it is possible to obtain a multi-level (more thantwo levels) modulator, without high frequency common mode components onthe output.

In FIG. 11, N (where N is an integer) SCOM modulators are synchronizedby an additional synchronization signal. This synchronization signal canbe any signal shape but preferably triangular, square or sinusoidal witha frequency of the wanted idle switching frequency. The synchronizationmeans can be any one of those described above.

In FIG. 12, N (where N is an integer) COM modulators are synchronized bytheir common COM signals. The common COM synchronization signal can beany signal shape with the frequency of the wanted idle switchingfrequency. The synchronization means can be any one of those describedabove.

The SCOM modulator can be implemented in any given AC-AC, DC-DC, AC-DCor DC-AC power conversion system, in particular a high precision DC-ACaudio power conversion system where the power stage elements operates ineither “on” or “off” state.

1. A synchronized controlled oscillation modulator (SCOM), comprising:at least one controlled oscillation modulator (COM) having anon-hysteresis comparator, a switching stage and a negative feedbackloop adapted to achieve conditions for controlled oscillation of apredetermined frequency by means of at least two poles, and asynchronizing device connected to said COM modulator.
 2. A modulatoraccording to claim 1, wherein the synchronizing device comprises anoscillation signal generator.
 3. A modulator according to claim 1,wherein the modulator is synchronized by another SCOM modulatorcomprising any oscillating modulator signal with the frequency of awanted idle switching frequency.
 4. A modulator according to claim 1,comprising a second COM modulator, said synchronizing device beingconnected between forward paths of said COM modulators and arranged tosynchronize the COM modulators with each other.
 5. A modulator accordingto claim 3, wherein said synchronization device comprises a circuitconsisting of R, RC or C components.
 6. A modulator according to claim5, wherein said synchronizing device comprises: a first and a secondseries resistances (RA, RB), a parallel resistor (Rosc), and a parallelcapacitor (Cosc), wherein said first series resistance is connected to afirst end of the parallel capacitor and parallel resistor, and saidsecond series resistance is connected to a second end of the parallelcapacitor and parallel resistor, and wherein said first and secondseries resistances are connected to a comparator in the forward path ofeach modulator respectively.
 7. A modulator according to claim 3,wherein said-synchronization-device comprises an active circuit,preferably including at least one high pass filter.
 8. A modulatoraccording to claim 1, wherein the modulator is extended by an additionalSCOM modulator driven in a full-bridge configuration to achieve athree-level pulse output.
 9. A modulator according to claim 8, whereinthe modulator is implemented in a PSCPWM system, without common modehigh frequency spectral contributions in the three level pulse output.10. A modulator according to claim 1, wherein the modulator isimplemented in a multi-loop MECC (N,M) control system for enhanced noisesuppression, where N and M are integers representing the number of localand global control loops respectively.
 11. A modulator according toclaim 1, further comprising N COM modulators synchronized by anadditional synchronization signal or by a common COM signal of said COMmodulators.
 12. A modulator according to claim 1, further comprising alimiter device to control the PWM modulation depth.
 13. A modulatoraccording to claim 1, wherein the modulator is implemented in a generalpower conversion system, in the general power conversion system is in aDC-AC audio power conversion system.
 14. A modulator according to claim1, wherein the modulator is used to drive an electrodynamic transducerload directly.